Fluorescein is a molecule reported in 1871, and has been widely used as a pH indicator or a labeling dye because of the high water solubility and high fluorescence quantum yield thereof. Since a calcium probe containing fluorescein as a mother nucleus was developed, there have been provided a large number of highly sensitive fluorescent off/on type probes utilizing intramolecular photoinduced electron transfer (PET), decyclization or cyclization of spiro ring, and the like. However, plural dyes containing fluorescein as a parent compound cannot be simultaneously used in molecular imaging, since fluorescence wavelengths thereof overlap with each other. Moreover, the probes utilizing the intramolecular photoinduced electron transfer suffer from a problem that such probes require precise design of the oxidation potential of the benzene ring, and therefore modification of the chemical structure is strictly limited.
Further, almost no reports were made as for structural modification of fluorescein at the oxygen atom of the 10-position of the xanthene ring, and optical characteristics of such compounds, wherein the oxygen atom at the 10-position of the xanthene ring is replaced with another type of atom, are not known so far. Although a compound corresponding to the basic structure of rhodamine, pyronin Y (PY), of which oxygen atom is replaced with silicon atom (TMDHS) and application of this compound as a fluorescent probe have already been reported (Best, Q et al., Pacifichem 2010, subject number 2335, Dec. 19, 2010; Yuichiro KOIDE et al., Fourth Convention of The Japanese Society for Molecular Imaging, subject number P8-9, May 14, 2009), this fluorescent probe having TMDHS as the basic structure is basically a probe utilizing the intramolecular photoinduced electron transfer or decyclization or cyclization of spiro ring. In addition, any compound corresponding to fluorescein, of which oxygen atom at the 10-position of the xanthene ring is replaced with silicon atom, has not been reported so far, and fluorescent characteristics of such a compound are also not known.